1. Field of the Invention
The present invention relates to a plastic lens and a process for making the plastic lens. More particularly, the invention relates to a process for making a plastic lens having urethane bonds or S-alkyl thiocarbamate bonds, and a lens produced by the process.
2. Description of the Prior Art
It is known to use diethylene glycol-bis-(allylcarbonate) (hereinafter referred to as DAC), PMMA and polycarbonate for preparing lenses. It is known to use an internal mold releasing agent such as butyl phosphate to improve the mold releasing properties upon molding these materials. However, internal molding agents are not required and result in impaired appearance of molded articles [Seiichi Mima, Polymer Digest, 3, 39 (1984); etc.].
Polyurethane base lenses exhibit high refractive indices in comparison with lenses made of the above-described materials. However, polyurethane adheres to a mold upon molding, and thus it is difficult to release the lens from the mold after polymerization. With this phenomenon in view, the present inventors previously proposed a process in which an external mold releasing agent is used (Japanese Patent Laid-Open No. 267316/1987, etc.) and a process in which a polyolefin resin mold is used (Japanese Patent Laid-Open No. 236818/1987) to improve mold releasing properties.
However, these foregoing processes are still insufficient in casting polymerization of polyurethane base lenses.
In a process in which an external mold releasing agent is used, a surface treating material on the inner surface of a mold is transferred to the surface or interior of the polymerized lens, thus producing irregularity or unevenness on the surface of the lens or turbidity within the lens. Further, whenever the mold is used repeatedly, it requires mold releasing treatment, thus making the process complicated for use in industrial applications and thus is very uneconomical due to the low productivity of the lens.
A polyolefin resin mold is deformed at elevated temperature so that the resulting molded lens has poor surface profile regularity. Thus, the lens is not suitable for use in applications in which a high degree of profile regularity is required.